Articles of carbon and graphite are conventionally produced by mixing a finely divided carbonaceous filler with a binder and extruding or molding the mixture. The carbonaceous filler may be a petroleum or coal-derived coke, powdered graphite, carbon flour or the like. The binder is usually a carbonaceous pitch derived from coal or petroleum. Pitches are also used as impregnants to fill the porosity of carbonized articles. Subsequent carbonization of impregnated carbons leads to higher density and strength. Pitches are a complex mixture of polynuclear aromatics generally derived from the thermal treatment of coal tar or a petroleum tar. Pitches are glass-like solids which gradually soften above room temperature and transform to a low viscosity fluid at elevated temperatures. The temperature at which pitch begins to exhibit liquid properties is referred to in the art as the softening point. The softening points of pitches can vary considerably depending on the selection of organic feedstock and subsequent hermal treatment. The softening point of pitch is usually determined by the Mettler procedure as defined in ASTM-D-3104-75.
For pitch to be useful as a binder or impregnant in the production of carbon or graphite products it should have the highest possible coke yield after carbonization and a softening point within a specified range. Coke yield generally increases with pitch softening point. The coke yield of a pitch is designated in the art as the Modified Conradson Carbon Value (MCC) and is determined using the procedure in ASTM-D-189. If the softening point of the pitch is too high, the processability of the carbon or graphite article is substantially affected. For example, in the production of graphite electrodes, the limitations in current commercial equipment limit the useful softening points of both binder and impregnant pitches to the range of about 90.degree.-120.degree. C. Electrode binder pitches with softening points in this range generally have MCC values below 60% while impregnant pitches with these softening points have MCC values of about 50% or less. Pitches with softening points below 90.degree. C. have low coking values. The use of low coking value pitches increases the porosity and depresses the density and strength of the carbon or graphite article made from such pitch. Although high softening point pitches with softening points in excess of 200.degree. C. and coking values as high as about 80% are commercially available, these materials are not currently useable in conventional carbon processes unless the softening point is reduced to about 120.degree. C. or lower. The pitch softening point may be reduced through the addition of a plasticizer. Materials currently used as plasticizers for pitch include petroleum and coal tar distillates, fatty acids such as oleic and stearic acid and others. Unfortunately, all of these materials have essentially no coking value and their use thus negates any benefit which might be expected from the use of a high softening point, high coking value, pitch.
A "plasticizer" has been discovered in accordance with the present invention for use in the manufacture of pitch having a higher carbon yield than heretofore attainable at a given softening point. For purposes of the present invention the term "plasticizer" means a material which is essentially totally soluble in pitch, has a high coking value and depresses the pitch viscosity to reduce its softening point. The present invention also encompasses a method of preparing a pitch for use in the manufacture of carbon and graphite products. Pitches made according to this invention have MCC values above at least a minimum of about 50% MCC and a preselected softening point within a range of between 90.degree.-120.degree. C.